A generic rotary machine is described for example in WO 00/28190 A1. In said publication, the rotary machine is a gas turbine of axial type of construction, having the commonly provided components of compressor, combustion chamber, turbine and a common rotor for the compressor and turbine. The known gas turbine comprises a compressor-side axial bearing with a main step and a secondary step for the axial positioning of the gas turbine rotor. The main step and secondary step may in this case bear against a side surface of a respective shaft collar so as to form a hydrodynamic slide film of hydraulic oil. Which of the two steps predefines the axial position of the rotor is in this case dependent on the operating point. A suitable hydraulic controller for this is known from EP 1 479 875 A1. Here, by means of a 4/2 directional valve, it is possible for either the main step or the secondary step to be pressurized depending on whether or not the rotor is to be displaced axially.
Alternative axial bearings to this emerge from WO 91/02174 A1 and from U.S. Pat. No. 5,795,073.
During intended operation of the gas turbine, the side surface of the first shaft collar bears against the main step of the axial bearing because the flow forces of the hot gas which act axially on the rotor in the region of the turbine are greater than the flow forces in the compressor, and thus displace the rotor constantly from the compressor in the direction of the turbine unit. The secondary step is situated in the force flow of the axial bearing instead of the main step when the rotor of the gas turbine is accelerated from a standstill to rated rotational speed. During said run-up, there is a resultant axially acting thrust force on the gas turbine rotor directed oppositely to the operational thrust force, specifically from the turbine unit in the direction of the compressor. Just before the rated rotational speed is reached, the present thrust direction changes abruptly, such that the axial positioning of the rotor then changes from the secondary step to the main step.
The already-known axial bearing is furthermore equipped with axially displaceable bearing elements in order for the rotor to be displaced counter to the flow direction of the hot gas, and thus for radial gaps in the turbine between the rotor blade tips and a housing wall situated opposite these to be minimized, during the steady-state operation of the gas turbine.
It has been found that, during the operation of the gas turbine, operating states may arise which lead to axial vibrations of the rotor. These axial vibrations can, in the worst case, lead to damage to the axial bearing or to components in the force flow if the amplitudes of the axial vibrations exceed a critical magnitude. The axial vibrations are usually caused by an unstable combustion process taking place within the combustion chamber. The causes of the instability are often manifold and not causally predictable.